In tumors, it is known that nitric oxide (NO) is present as a vasodilator and regulates the blood flow which is crucial to tumor growth. Since NO is present in both tumor and normal cells in macrophage, neuronal, and endothelial roles, it is desired to determine if NO could have an effect on tumor cell growth outside of a vascular system. This blood-vessel-free environment has been established using a multicell spheroid model of the colon carcinoma, SW 480. Preliminary experiments have shown a decrease in SW 480 growth with exposure to the NO synthase (NOS) inhibitor, L-NAME. Recording the DNA content of these cells with and without L-NAME exposure using a flow cytometer is a subsequent step in determining the effect of NO in the cell cycle of this tumor. The DNA will be labelled with bromo deoxyuridine (BrdU) and fluorescently stained with Hoerchst 33342 and mithramycin.